Magnetic resonance imaging (MRI) has proven useful in the diagnosis of many diseases such as hepatic steatosis, cancer, multiple sclerosis, sports related injury, and bone marrow disorders. MRI provides unique imaging capabilities which are not attainable in any other imaging method. For example, MRI can provide detailed images of soft tissues, abnormal tissues such as tumors, and other structures which cannot be readily imaged using techniques like X-rays. Further, MRI operates without exposing patients to ionizing radiation experienced in X-rays. For these and other reasons, MRI is commonly utilized in the medical field.
In comparison to other imaging modalities, MRI is unique in that the MRI signal is represented by a complex number, rather than simply a scalar (such as X-ray attenuation in CT). The image value for each image pixel, therefore, usually includes a magnitude and a phase. Although the phase of an image pixel may carry important information and may be used in many applications such as chemical shift imaging, thermal imaging, and blood flow quantitation, it is usually discarded in the standard image reconstruction process. The underlying reason is that some unwanted error phase almost always accompanies the desired phase. Although many methods have been developed to remove the error phase, a truly reliable and automated phase correction method is still lacking.
A clinically relevant application where correction of phase errors is important is the Dixon chemical shift imaging. In MRI, the signal-emitting protons may resonate at different Larmor frequencies because they have different local molecular environments or chemical shifts. The two most distinct species found in the human body that are signal-generating for MRI are water and fat, whose Larmor or resonant frequencies are separated by about 3.5 ppm (parts per million). Typically, both water and fat will appear in an MRI image. However, in many clinical MRI applications, it is desirable to suppress signals from fat because they are usually very bright and can obscure lesions. In some clinical applications, detection and quantitation of fat may also be important. Presently, the most commonly used method for fat suppression is the chemical shift selective saturation (CHESS) method, which, despite its many advantages, is known to be intrinsically susceptible to both the radiofrequency (RF) and the magnetic field inhomogeneity. Another technique that is sometimes used for fat suppression is the short tau inversion recovery (STIR), which is based on the characteristically short T1 relaxation constant for fat, rather than on its Larmor frequency. The drawbacks of STIR include reduction in scan time efficiency and signal-to-noise ratio as well as potential alteration to the image contrast.
Several phase error correction methods have been previously published or disclosed for separating water and fat signals in MRI. In general, these methods require multiple input images with varying but specific relative water and fat phase angles. For example, in one implementation of the three-point Dixon method, three input images with a relative water and fat phase angle of 0, 180°, 360°, respectively are acquired [7-8]. In another implementation of the three-point Dixon method, three input images with a relative water and fat phase angle of 0, 90°, 180°, respectively are acquired [11]. In one implementation of the two-point Dixon method, two input images with a relative water and fat phase angle of 0 and 180°, respectively are acquired [6]. Acquisition of the multiple input images usually leads to at least doubling (in the two-point Dixon method) or tripling (in the three-point Dixon method) of the minimum total scan time. A recently disclosed phase correction method allows efficient and robust water and fat separation using two-point Dixon data when water and fat signals are 0 and 180°, respectively. When the two-point Dixon data are acquired in a dual echo with readout gradients of alternating polarity after one RF excitation, the total scan time is greatly reduced relative to that of a conventional two-point Dixon method and becomes comparable to that of a single acquisition scan. Nonetheless, two input images with specific water and fat relative phase angles (one in-phase and one 180° out of phase) are still required. Often times, the in-phase and 180° out of phase angles required in a two-point Dixon acquisition would leave out some deadtime during the TR time and cause some inflexibility in choosing the imaging parameters (e.g., the receiver bandwidth and the frequency encoding matrix size). Consequently, the true minimum scan time allowed by the system hardware and user-selected imaging parameters is not realized and the image quality is not optimized.
The referenced need for phase error corrections in MRI and shortcomings of some of the existing approaches as discussed above are not intended to be exhaustive, but rather are among many that tend to impair the effectiveness of previously known techniques concerning image data acquisition and image reconstruction; however, those mentioned here are sufficient to demonstrate that the methodologies appearing in the art have not been satisfactory and that a significant need exists for the techniques described and claimed in this disclosure.
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While the present disclosure is susceptible to various modifications and alternative forms, specific example embodiments have been shown in the figures and are herein described in more detail. It should be understood, however, that the description of specific example embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, this disclosure is to cover all modifications and equivalents as illustrated, in part, by the appended claims.